This manuscript describes a series of environmental chamber experiments designed to probe the effects of seed aerosol and ammonium on secondary organic aerosol formation from acetone. By starting with different seed aerosols, the authors are able to investigate the impacts of particle acidity on SOA formations -- although this is hard to separate from chemical composition, i.e., the availability of ammonium for heterogeneous reactions. To separate these effects, the authors then add ammonia to the chamber after turning off the lights, and find that ammonium availability enables formation of C-N bonds and light-absorbing compounds in the aerosol. Finally, the authors extrapolate the SOA yields observed in the chamber -- despite the fact that the oxidation and seed particle conditions in the chamber are vastly different from those in the atmosphere -- to calculate a rough global SOA formation from acetone and contribution of acetone to the SOA budget.

The manuscript flows well logically, with straightforward descriptions of the experiments, and the figures are clear and easy to understand. The authors' comprehensive modeling of wall losses -- not just of particles but of ammonia and organic vapours -- is a big plus. However, I believe some shortcomings with the manuscript require major attention before this paper can be publication-worthy -- most notably, some analysis of the effects of photolysis in the chamber, discussion and consideration of the RO2 radical fate in the chamber, comparisons to other methylglyoxal chamber/modeling studies, and a clarification of assumptions (or, better yet, a more nuanced approach) behind the calculation of acetone's contribution to the global budget. More detail on all of these are included in the line-by-line comments below.

L 63: It's not clear what's meant by "OAs" here (the acronym hasn't yet been defined, but seems reasonable to assume it's organic aerosol -- but even then I'm not sure what "all the OAs" in this case means).

L 68: also not sure what "the uptake ... is of salting-in/salting-out effects" is supposed to mean. That uptake is *affected by* salting in and out?

L 70: It's not entirely clear what's meant by "highly volatile" VOCs -- i.e., what's the volatility cutoff that counts as "highly" volatile? Many species that are widely considered volatile are indeed included as SOA sources in models, such as glyoxal and methylglyoxal (see, e.g., Fu et al., 2008, DOI: 10.1029/2007JD009505 for GEOS-Chem) and even formaldehyde (see, e.g., Moch et al., 2020, DOI: 10.1029/2020JD032706 and Dovrou et al., 2022, DOI: 10.1073/pnas.2113265119). In fact, this likely overlaps with the proposed acetone SOA source estimated here, since acetone predominantly makes SOA via methylglyoxal. It would be worth comparing your results here to previous model estimates of methylglyoxal SOA.

L 77-83: it's disingenuous to suggest that SOA formation from acetone hasn't been studied before -- in fact it's been the subject of a lot of study, but those studies usually started with methjylglyoxal instead of acetone. SOA formation from methylglyoxal, and the impacts on that process of heterogeneous reactions and aerosol properties, has been extensively characterized in the past -- see, e.g., De Haan 2018 (DOI 10.1021/bk-2018-1299.ch008), Tan et al. 2010 (DOI: 10.1016/j.atmosenv.2010.08.045), and Schwier et al. 2010 (DOI: 10.1021/es101225q) among many others. Starting with methylglyoxal is a reasonable tactic considering that acetone photooxidation in the atmosphere produces methylglyoxal in high yields and it's widely acknowledged (e.g. in your own Figure 6) that methylglyoxal is the dominant intermediate in SOA formation from acetone. Previous work on SOA formation from methylglyoxal should be acknowledged and summarized here in the intro, and compared to your results later in the manuscript.

L 113: Does acetone photolyze sufficiently quickly with the 254nm lights for this to be an appreciable loss process during the photooxidation stage? This should be modeled, ideally with a box model of the chamber that also accounts for the self-limiting effect of H2O2 as a source of OH radicals (high concentrations of H2O2 needed for OH production also serve as a strong OH sink through the OH + H2O2 reaction, limiting the concentration of OH that can be sustained). If acetone photolysis is indeed a factor in its loss in the chamber, what effect would this have on your conclusions?

L 120: The values in Table S1 suggest that these experiments had sustained OH and starting acetone concentrations much higher than would be representative of the atmosphere. First of all, how much H2O2 was used to achieve this? Second, what does this mean for the fate (i.e. reactive pathways) of the peroxy radicals formed by acetone + OH (+ O2)? I would imagine this high VOC and OH mixture would create very high peroxy radical (RO2) concentrations, which would then react predominantly via RO2 + RO2 chemistry, although that's only a minor pathway in the troposphere. This could mean you're producing a lot of SOA from dimers that wouldn't have formed if the acetone-RO2 were reacting with HO2 or NO -- the dominant atmospheric pathways. Can a box model or other method be used to estimate the relative contributions of these pathways?

L 141: were gamma(H+) and m(H+) also calculated by E-AIM?

L 162: need to spell out methylglyoxal before abbreviating it (MGly)

L 173-176: how was the N/C ratio calculated -- it looks like only N in organic fragments was considered, since otherwise the ratio would've started out extremely high considering all the ammonium in the particles? Is it possible that N-containing organic fragments are made in the AMS (i.e. a reaction during the heating and discharging) rather than being representative of the aerosols in the chamber?

L 240: this difference in acid partitioning between the different pH seed aerosols does seem a highly likely effect; can it be estimated from the differences in measured gas-phase concentrations of formic and acetic acids between the various experiments?

L 256-260: couldn't it just be that the higher-order oligomers fail to form in the neutral aerosols, since the condensation reactions that produce them are acid-catalyzed and will preferentially occur on the acidic seeds? It doesn't seem like there is a need to invoke hydroperoxide decomposition here.

L 351-366: this method of calculating acetone's SOA formation and contribution to global SOA production effectively assumes that all acetone in the atmosphere (a) follows gas-phase photooxidation reactive pathways similar to those experienced in these chamber experiments, which seems unlikely given the high RO2 and OH concentrations in the chamber, and (b) is exposed to the same high inorganic seed particle loadings that exist in this chamber, which is not the case. In the same way you normalized the SOA formation to surface area in Figure S4, estimates of ambient SOA formation should be scaled by the ambient aerosol surface area that acetone (or more accurately its photooxidation products) actually sees in the atmosphere, which would likely result in a far smaller contribution to the global SOA budget. Without these adjustments the rough yields and budgets estimated here should not be reported as headline numbers in the abstract, since they are likely to strongly overestimate the contribution of acetone. In fact, once these assumptions are reconsidered, the estimates might be more in line with the "irreversible uptake" of methylglyoxal used in models.

Secondary organic aerosols are the major component of fine particles in the atmosphere and its evolution cannot be predicted by current numeric models due to some missing heterogeneous formation mechanisms. Zhang and co-authors investigated the heterogeneous reaction dynamics of SOA formation from the photochemical oxidation of acetone via chamber experiments and analyzed the effects of seeds and NH₃ on the formation of SOA and BrC, finding that no BrC formed on neutral particles although in the presence of NH₃ and only considering the role of MGly will inevitably underestimate the contribution of acetone to SOA production in the atmosphere. The research is interesting and would be helpful for improving our understanding on SOA and BrC formation mechanism. And the wall loss corrected of particles and volatile compound (NH₃ and VOCs) is detailed and systematic. This paper is well organized and the figures are clear. I can recommend its publication if the following issues can be addressed.

Specific comments:

1. In experiments, the photodissociation of acetone also can take place under 254 nm UV irradiating conditions at Phase I. Therefore, the influence of the photodissociation of acetone on the SOA formation should be considered.
2. In different experiments with various inorganic seeds, can the reactions in gas phase be affected by the physicochemical properties of seeds? The authors should discuss the difference of the reactions in gas phase in the presence of different inorganic aerosols, including the differences in gaseous products concentrations.
3. Line 213, authors proposed that methylglyoxal is the key species in SOA formation from acetone. However, other gaseous products (e.g. acetone alcohol and organic acids) can also contribute to the SOA formation in aqueous phase. Their contribution on SOA formation and corresponding reaction pathways should be considered.
4. In section 3.2, RO2 fate can influence the SOA formation processes obviously, therefore, can the particle acidity and NH₃ change the RO2 chemistry and further affect the SOA formation?
5. Line 277, the partitioning coefficients of NH₃ on different seeds were estimated using the eq. S15 and S16 (Guo et al., 2017; Nah et al., 2018). The authors can compare the theoretical values with the actual values observed in experiments.